[🚀esl-carousel]: update carousel Public API

[ala-n]

As an ESL architect, I want to be sure that the carousel component have simple and powerful enough API to control carousel state

Primary API points and methods:

• State markers + configurable state classes
• A11ty attributes managed for slides and carousel in general
• Should consist of the main esl-carousel component, mandatory esl-carousel-slide (esl-carousel-item) inner component, and optional carousel plugins components
• Should dispatch event hooks for the following stages:
  • before slide change
  • after slide change
  • configuration change
• goTo(position, direction) (name is discussable) method to change carousel active alides
• next/prev shortcut functions to move carousel back and forward
• size read-only property that describes the number of inner slides (items)
• $slides readonly slides accessor
• sceneSize - read-only count of visible slides, configurable using config property
• index - read-only first active slide index
• $activeSlides - array of the active slide elements
• $activeSlide - first active slide